Infrared imaging systems are often fabricated as two dimensional focal plane arrays. Focal plane arrays often form images of slowly-varying schemes. To enhance the viewing of these schemes, longer frame rate integration is desired. "TV frame rates" eg of 40 ms are preferable.
The present inventors have found that temporally modulating or chopping the infrared scene in conjugation with AC coupling of the detector signal obtains TV rate integration, and also reduces .sup.1 /.sub.f noise problems.
Different kinds of infrared photo detectors are known. A GaAs/AlGaAs quantum well photo detector has been used to trap electrons in a low potential energy well--often called a quantum well. However, the quantum well infrared photo detectors previously used have had inherently large and spatially nonuniform dark currents. This leads to saturation of the readout circuits and hence limits the integration times. The spatial nonuniformity of the dark currents also prevents us from using a constant or spatial subtractance scheme. To obviate this problem, the present inventors have recognized that temporal modulating produced the significant advantages of .sup.1 /.sub.f noise reduction and independence of the large dark current nonuniformity.
Simple scanning IR imaging systems with a small number of detectors may be individually addressed by external electronics. Infrared two dimensional focal plane arrays, however, usually have between 10.sup.4 and 10.sup.6 elements, each of which receive an IR input to provide an IR signal indicative of a scene. Therefore, individual connections become impractical. The array may therefore be hybridized via Indium bumps to a silicon readout array. The performance of these arrays are still limited, however, by the charge integration capacity of the individual capacitors.
The typical readout circuits can handle roughly 10.sup.7 electrons for each 10.sup.-5 cm.sup.2 pixel. Therefore, there is insufficient charge handling capacity to allow TV rate integration of the total quantum well infrared photo detector current, which is several 10.sup.-4 A/cm.sup.2 .apprxeq.on the order of 10.sup.10 electrons/pixel/sec. This is important since it indicates how long it takes pixel to saturate--typically less than 1 ms. This problem thus has tended to minimize the advantage of staring over scanning arrays since it has limited the integration time or limited the bandwidth since integration time is proportional to .sup.1 /.sub.f. Hence, this also has reduced the noise-equivalent temperature differences. Large integrated voltages on the readout capacitor have also tended to drive the injection transistor in an unintended way and possibly induce a nonlinear detector response. The integration cannot be done off-chip using today's technology since such external sampling and averaging would require prohibitively fast electronics and hence high cooling requirements.
The inventors have found that modulating or chopping the incident radiation makes it possible to separate the `ac` type photo-response from the `dc` type dark current. The dark current is the detector's response to the modulator being off--that is without photon flux of the scheme impinging on the modulator. If the dark current is subtracted from the detector's signal with the modulator on (signal plus dark current) the result represents the net current added to the noise accumulating on the capacitor. This system can allow an IR-modulator along with an electronic circuit to collect the detector's signal current only, and hence can increase the integration time. By increasing the integration time by a factor of N, the signal to noise ratio is increased by a factor of .sqroot.N/2.
Even if the charge integration limitations are somehow removed by reducing the dark current, the temporal modulation reduces the .sup.1 /.sub.f noise by shifting the central bandwidth from near `dc` to the modulator's frequency.
Mechanical choppers have been used in some currently developed infrared focal plane arrays. However, these require a fast-moving mechanical component with associated power requirements. They also produce associated operating noise and may have limited reliability.
It is one objective of the present invention to develop a fast, reliable electro-optic infra red ("IR") modulator to help realize the full potential of IR-focal plane arrays. This is effected according to the present invention by monolithically integrating materials which form an infrared-focal plane array with a modulator. Preferably, similar materials are used for both.